Electrical aspects of flame quenching

Abstract

Carbon monoxide distributions within small laminar natural gas flames are studied by micro-probe sampling and by numerical simulation, at progressively increasing proportions of air added to a constant fuel flow. The results are compared with CO emission to the atmosphere when such flames are quenched at various heights above the burner – by a variety of solid surfaces – with a view to quantifying the hazard associated with, for example, domestic boilers. Numerical simulations of small flames support the experimental results. The electrical structure of quenched flames under small (<10V/mm) electric fields is investigated, with the ultimate objective of devising a warning system signaling the onset of a quenching hazard. The variable electric field, derived from batteries and a potential divider, is applied between the burner and a copper grid – one of the quenching bodies – recording current as a function of height, for both polarities. Resistance/distance is of the order 108Ω/mm, depending on local temperature and composition. Because of their low mobilities, positive ions contribute only over very small distances; for longer path lengths the flame acts as a rectifier. Accordingly, electron conduction is utilized to detect the proximity of a quenching surface to the flame well before any hazardous release of CO escape could arise. The contours of first detection around the flame – for a current threshold of 10−8A – lie substantially outside the regions in which CO is detected in quenching products for both a natural gas diffusion flame and near-stoichiometric premixed flames.